Fuel injection systems deliver fuel to the combustion chamber of a gas turbine engine, where the fuel is mixed with air before combustion. One form of fuel injection system well-known in the art utilises fuel spray nozzles. These atomise the fuel to ensure its rapid evaporation and burning when mixed with air. Typical fuel injectors have a fuel spray nozzle containing one or more fuel discharge orifices, and a feed arm which extends from the nozzle to one or more fuel supplies (e.g. engine fuel manifolds). Passages in the feed arm and the nozzle transport the fuel from the supplies to the discharge orifices. An annular combustor usually has a circumferential arrangement of such fuel injectors.
One type of fuel injector has an airspray nozzle in which fuel delivered to the combustion chamber by one or more fuel discharge orifices in the nozzle is aerated by air circuits carrying air discharged from a compressor of the engine. Swirlers in the air circuits can ensure rapid mixing of fuel and air at the discharge orifices, to create a finely atomised fuel spray. More specifically, the swirlers impart a swirling motion to the air passing therethrough, so as to create a high level of shear and hence acceleration of low velocity fuel films.
Another type of fuel injector is a pressure-jet injector in which the fuel is passed through a swirl chamber and thence to a discharge orifice, where the fuel is atomised to form a cone-shape spray. The rate of swirl and the pressure of the fuel determine the extent of atomisation.
Multi-stage combustors are used particularly in lean burn fuel systems of gas turbine engines to reduce unwanted emissions while maintaining thermal efficiency and flame stability. Thus duplex systems have pilot and mains fuel manifolds feeding pilot and mains fuel discharge orifices respectively of each fuel spray nozzle. In such combustors the input of fuel is “staged”: a pilot flow of fuel through the pilot discharge orifice being used for low power operation and a mains flow of fuel through the mains discharge orifice being brought in additionally for higher power operation. The fuel for the manifolds typically derives from a pumped and metered supply. A splitter valve can then be provided to selectively split the metered supply between the manifolds as required for a given staging.
Within fuel injectors of such a duplex system, check/distribution valves, known as a fuel flow scheduling valves (FSVs), are typically associated with the feed arms so that when a mains stage is de-staged, the valves provide a drip tight seal preventing mains fuel from leaking into the injector. A problem can arise, however, in that the mains fuel left in the mains passages of the injectors when the mains passage is not flowing is stagnant. In this state, heat soaking into the injector (e.g. from the surrounding high temperature air discharged by the engine compressor, and/or radiated from the combustor) can cause the fuel to heat up and react, leading to coking in the passages. Thus controlling the temperature of the fuel inside the passages is important. On the other hand, the higher the temperature at which fuel is burnt in the combustor, the higher the efficiency of the combustion process and the greater the engine performance.
It would be desirable to provide a fuel injector that improves management of these conflicting requirements.